It's simple.. Nintendo made 1st party games so polished that 3rd party companies just couldn't compete with them. So most didn't even try.
That's been the case ever since 3rd parties abandoned the N64. Yeah there's some decent 3rd party stuff on N64, GC, and Wii, but the Nintendo stuff is generally regarded as the better stuff and the reason you buy the machines - not so you can play RE4.
Time will be when the broadest river dries
And the great cities wane and last descend
Into the dust, for all things have an end